Combine the roasted red peppers and the jar liquid in a blender or food processor and puree until smooth; set aside.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook, stirring occasionally, until soft, about 7–10 minutes.
Add the minced garlic to the skillet and cook for 1 minute, then add the sliced mushrooms and continue cooking, stirring occasionally, until the mushrooms are softened, about 10 minutes.
Stir the pureed roasted peppers into the mushroom mixture, then add salt, black pepper, dried Italian herbs, and red pepper flakes (if using). Mix to combine.
Add the calamari rings to the skillet, stir to coat, and simmer in the sauce until the calamari are tender, about 20–30 minutes.
While the calamari cooks, bring a large pot of salted water to a boil and cook the fettuccine according to package instructions. Drain and rinse with cold water, then set aside.
Stir the Half & Half (or heavy cream) into the calamari sauce and remove the skillet from the heat.
Add the cooked fettuccine to the skillet and toss gently to combine and coat the pasta with the sauce. Garnish with chopped parsley and serve.